Dow 7 Chemical Conversion Coating for Magnesium:

Introduction:

Magnesium and its alloys are more galvanically active than even zinc on the galvanic series.  Magnesium will form a strong galvanic cell with every other metal it comes into contact with and is always sacrificial. 

Therefore, these alloys must be properly protected from corrosion damage.

Background:

Dow 7 is a good choice for both new and overhauled parts that have dissimilar materials such as inserts, or where conditions will tolerate a less robust and less costly alternative to Dow 17 anodizing.  It makes

an excellent paint base and provides the best corrosion protection of the available chemical coatings for magnesium. 

Dow 7 is a chemical conversion coating which results in no appreciable dimensional change to the component.  The surface of components coated with Dow 7 is changed to a brassy or dark brown finish depending on the alloy.  Note that this coating film is dielectric (non-conductive). 

Specification & Certifications:

The coating produced by proper application of the contents of this solution conforms to SAE AMS-M-3171, Type III (formerly MIL-M-3171, Type III). 

Exceptions:

The Dow 7 treatment provides a protective coating on all standard magnesium alloys.  However, it is

not for use on the following magnesium alloys: EK30A, EK41A, HM31A, HM21A, HK31A, La141A and M1A.  Due to their chemistry, no coatings form on these particular alloys.

Operating Limitations of the Dow 7 Coating:

Extended exposure of the developed coating to operating environments at elevated operating temperatures in excess of 150ºF will dehydrate the coating resulting in reduced corrosion protection.

Process Steps:

The complete process requires two steps.  The first step is an activator immersion which is available as either Type I or Type II.  This step is performed at ambient temperature in the range between 70ºF and 90ºF.  This is followed by a deionized water rinse.  The second step is the immersion into the chromate bath which must be at boiling point.  This is also followed by a deionized water rinse and air dry.

Step 1A—Surface Activation For Wrought Alloys & Weldments:

Activator Type I is formulated specifically for use with the “Dow 7”conversion coating process for magnesium alloys.  Type I is chosen for most wrought alloys, hog-outs, and weldments.  Being less aggressive than Type II, it is formulated as the safer and easier-to-use activator option when the aggressiveness of the Type II activator to overcome die casting skin is not required.               

Application Notes:

Parts containing bearings, studs, and inserts of brass, bronze, steel, cadmium-plated steel and aluminum may be treated using this activator. 

Precautions:

This formulation contains fluorides that are highly corrosive to skin and bones.  Use only with appropriate personal protective equipment (PPE) which includes neoprene waders, a chemical apron, neoprene gloves,

face shield, and either a fume hood or other suitable ventilation.

Process:

This Type I activator kit includes a 1-Gallon jug of Type I concentrate packed in a 5-gallon screw top pail.  At time of use, 3 gallons of water are added to the pail to which the 1-gallon jug of Type I activator is added.  This will make a 4-gallon immersion bath which is generally large enough to handle most small

parts.  Immersion time is generally about 5 minutes in duration.  When the process is complete, simply screw the lid on the pail and store for future use.  This step in the process is followed by a deionized water rinse.

Step 1B—Alternate Surface Activation For Die Cast Alloys:

This is Activator Type II formulated specifically for use with the “Dow 7”conversion coating process for magnesium casting alloys.  Type II is chosen for use on die cast alloys.  Being more aggressive than Type I, it is formulated to remove the die casting skin that naturally forms on cooling.              

Application Notes:

Castings containing bearings, studs, and inserts of brass, bronze, steel and cadmium-plated steel may be treated using this activator.  Note that aluminum inserts will be rapidly attacked by this activator.

Precautions:

This formulation contains Hydrofluoric Acid that is highly corrosive to skin and bones.  Use only with appropriate personal protective equipment (PPE) which includes neoprene waders, a chemical apron, neoprene gloves, face shield, and either a fume hood or other suitable ventilation.

**HF is the most dangerous industrial acid in existence**

It attacks calcium in bones.  Strong irritant to eyes and skin.  Highly corrosive to skin and mucous membranes.  Toxic by inhalation and ingestion.  Will attack glass specifically and generally any silicon-containing materials.  Read, and fully understand the risks and precautions associated with this substance before using.

Those using Hydrofluoric Acid or products containing it should have a supply of Calgonate® Calcium Gluconate antidote gel readily available in case of accidental exposure.

Process:

This Type II activator kit includes a 3-liter jug of Type II concentrate packed in a 5-gallon screw top pail.  At time of use, 3.3 gallons of water are added to the pail to which the 3-liter jug of Type II activator is added.  This will make just over 4-gallons of immersion bath which is generally large enough to handle

most small parts.  Immersion time generally ranges from 30 seconds to about 5 minutes in duration.  When the process is complete, simply screw the lid on the pail and store for future use.  This step in the process is followed by a deionized water rinse.

Included with this kit is a tube of Calgonate® Calcium Gluconate antidote gel for emergency use in case of skin contact.

Step 2—Dow 7 Chromate Immersion:

The second step will require a stainless steel vessel that will allow the heating of the Dow 7 solution to boiling point (200ºF).  The chemical make-up powder comes packed in a 1-gallon can for dilution with 4-gallons of water.  Since it is difficult to assure complete mixing of powders at the powder stage, contents of can must be used in its entirety to assure the proper proportion of its constituents.  Bath can be checked to be in the range of pH 4.1-5.6.

Immersion time for the parts is about 30 minutes, also followed by a water rinse before allowing to hang dry for 24 hours.  Drying by circulating air at 160º- 200ºF will produce a dark brown coating of increased hardness and reduced solubility.